[WalterGR]

    It's not a solved problem if your project is gigantic
    or if you're doing cross-platform development and don't
    want to or can't use the build system of the IDE or if
    it's C++.

I can't comment on whether the problem is solved for C++ because I haven't done C++ development in several years. But, for the other potential roadblocks you listed: no, they're all solved.

    To me solving the problem has a higher bar - you should
    be able to open a large project for the first time and
    have usable intelligent editor support within a second.

Well, therein lies the rub. With the "intelligence on a novel project within one second" requirement, if it hasn't already been solved then you will never find it to be.

Many people are fine with a tool that has a large(r) start-up cost if saves them time (and/or headache) during run-time. Also, I don't think - though I can't prove - that it's common for people to open and close large projects in IDEs often enough that start-up time is a primary concern.

Combine that with increased tool feature count and project complexity as time goes on, as well as limited resources for tool development, plus the current start-up times being Good Enough, and the result is a problem that won't get solved.

    [IDEs] require you to use their build system...

I'm not sure what you mean. In my experience, you can use IDEs as glorified text editors - and build from the command line or some other external build tool.